Principles of Inheritance

1. Principles of Inheritance GENETICS

2. DNA • found in nucleus of each cell • composes chromosomes • chromosomes contain genes • genes are biological blueprints • dictate how we look, how our body functions & may be even how we behave • traits are inherited • passed down from generations before us • science of heredity is genetics

3. Genetics • the idea of traits being inherited has been around since time of ancient Greek philosophers • modern science of genetics did not begin until 1860 • Gregor Mendel • Father of Genetics • helped lay down principles of modern genetics • Central European monk • conducted experiments using garden peas • ideas were published in 1860's but were unrecognized until after his death • not appreciated until early 1900s • work applies to humans as well as peas • illustrates basic rules of inheritance

4. Rules of Inheritance • Mendel discovered basic genetic principles breeding garden pea plant • exercised strict control over mating of these plants • studied seven characteristics • each with two possible forms • flower color-purple or white • seed color-yellow or green • flower position-axil or terminal • pod shape-inflated or constricted • stem length-long or short • pod color-yellow or green • seed shape-round or wrinkled

5. Rules of Inheritance • most important conclusion was • inherited variations are transmitted to offspring as discrete units • until this time most assumed characteristics of individual organisms were blended from generation to generation • particulate theory • Particles-now known as genes GENE

6. True Breeding Plants • before beginning Mendel worked with his plants to ensure he had true-breeding plants • produce offspring that are identical to parents • purple flowers purple offspring

7. Hybridization-Cross-Breeding • purple mom + white dad • hybridization • or simply a cross • offspring are hybrids

8. Cross-Breeding • true breeding parents-P generation • for parental • children-F1 generation • f=filial-Latin for son • when F1 plants are matedoffspring-F2 generation

9. Mendel’s Experiments • Mendel noticed that traits were transmitted in predictable ways from parents to offspring • crossed different strains of purebred plants & studied their progeny • at first worked with consequences of crossing one trait at a time • monohybrid cross • would cross purple plant with white plant & look at color of offspring • F1 generation of this cross was always purple • Mendel wondered what had happened to heritable factor for white

10. Mendel’s Experiments • when crossed F1 generations • missing white factor reappeared • 75% of offspring had purple flowers • 25% had white flowers • 3:1 ratio

11. Mendel’s Experiments • same pattern of inheritance was found for all characteristics of pea plant • in cross-pollinating green pods-first offspring generation (f1) always had green pods • f2generation consistently had 3:1 ratio of green to yellow

13. Mendel’s Conclusions • white or yellow genes do not disappear in f1 generation • masked by purple or green gene • individuals inherit one unit from each parent for each trait • specific trait may not show up in an individual • may be passed to next generation • from his results, Mendel described four specific hypotheses

14. Mendel’s Hypotheses • there are alternative forms of genes-alleles • for each inherited characteristic an organism must have 2 genes • one from each parent • maybe the same or different • two of same allele- homozygous • two different alleles-heterozygous

15. Mendel’s Hypotheses • alleles represent genotype • when alleles are differentallele that determines appearance (phenotype) is dominant • other allele has no observable effect on phenotype-recessive • dominant genes are always expressed • need only one dominant gene to have a particular phenotype • to have recessive characteristic individual must carry two recessive genes • unlessgene islocated on a sex chromosome • customary to use capital letters for dominant traits • small letters for recessive ones

16. Genotype & Phenotype • brown eye color is dominant (B) • blue (b) is recessive • person with genotype BB or Bbwould have brown eyes • person with genotype bb would have blue ones

17. Law of Segregation • each f1 generation plant inherits one allele from one parent & one allele from the other • when f1 plants mated, each allele had an equal chance of being passed on to offspring • for any particular trait, a pair of alleles from each parent separate • only one allelepasses from each parent tooffspring • which allele in a parent's pair is inherited is a matter of chance

18. Law of Segregation • genes occur in pairs because chromosomes occur in pairs • during gamete production-members of each gene pair separate so each gamete contains one member of a pair • during fertilization full number of chromosomes is restored • members of a gene or allele pair are reunited • segregation of alleles occurs during process of gamete formation-meiosis

20. PunnettSquare • used to illustrate basic rules of inheritance • shows alleles of mother and alleles of father • bysimple multiplication one can figure out probability of obtaining offspring with characteristics of parents

21. PunnettSquare

23. Examples

24. Example • Brown eyed father-BB • Blue eyes-mother-bb • Recessive trait

25. father with red hair recessivetrait has children with mother with black hair dominant trait probability of having children with red hair is ? each child would carry a gene for red hair this is the case if mother has two dominant alleles in her genotype what if we know that woman’s mother had red hair Example r r R Rr Rr Rr R Rr

26. Dihybrid Cross • Mendel next crossed & followed inheritance of two traits at same time • dihybird crosses

27. Dihydrid Crosses • two of the characteristics Mendel studied were seed shape & color • seeds were either green or yellow & either wrinkled or round • knew round & yellow were dominant • wrinkled & green were recessive • wondered what would happen in a dihybrid cross • mating GGWW pea with ggww one

29. Principle of Independent Assortment • f1 generation yielded heterozygous hybrids or RrYy • phenotype was round & yellow • when f1 generation was crossed found distribution of one pair of alleles into gametes did not influence distribution of other pair • genes controlling different traits are inherited independentlyof one another • Principle of IndependentAssortment • ratio was9:3:3:1 • 9 yellow, round, 3 green, round, 3 yellow, wrinkled and one completely recessive pea or green, wrinkled

31. PunnettSquare

33. Test Crosses • Used to determine the genotype of a specific specimen • suppose you wanted to determine genotype of a specific organism • you have a purple flowering pea plant • want to know if your pea plant has purple flowers because it is homozygous or heterozygous • unknown plant is mated with known plant • cross purple-flowered unknown with white-flowered plant (completely recessive) • by counting individuals exhibiting each of resulting phenotypes, we could know genotype of unknown • if all offspring exhibited purple flowers we would conclude unknown parent is homozygous • if offspring exhibited 1:1 ratio of purple to white flowers, conclude unknown parent is heterozygous

34. Family Pedigrees • sometimes, it is possible to determine genotype by evaluating pedigrees • If you know traits of your parents & traits of your grandparents by using Mendelian principles you can predict possible phenotypes of your offspring • you can trace your family tree

35. Pedigrees

36. In Class Exercise C c CC Cc ? ? C • c • Cc • c c

37. Mendelian Pattern Inheritance • genes coding for a particular trait are located at particular positions on chromosomes-loci • come in several forms-alleles • receive one allele from each parent • if identical-homozygous for a trait • if different-heterozygous • recessive traits are not expressed in heterozygotes • for recessive alleles to be expressed, one must have 2 copies • dominant traits can be expressed in presence of another, different allele • dominant alleles prevent expression or mask recessive alleles in heterozygotes. • traits that are result of one set of genes are single gene traits • transmission of single gene traits follows Mendel’s patterns of inheritance

38. Other Patterns of Inheritance • over 4,500 human trains are known to be inherited according to simple Mendelian principles • there are exceptions to Mendel’s rules

39. Incomplete dominance • offspring is heterozygous for a trait but phenotype is intermediate between phenotypes of homozygous parents • heterozygous snapdragons of white & red parents have pink flowers • sickle cell disorder • homozygous individuals have either normal blood or sickle cell anemia while heterozygous individuals have sickle cell trait

40. Incomplete Dominance

41. Codominance • phenotypes for both alleles at a locus are expressed at same time • human ABO blood system shows both simple Mendelian inheritance & codominance • A & B alleles are dominant to O • if have genotype AOblood type is A • if BOblood type is B • however, neither A or B alleles are dominant to one another • Codominant-both traits are expressed • person with allele for A & one for B has blood type AB • OO = Blood type OAO = Blood type ABO = Blood type BAB = Blood type ABAA = Blood type ABB = Blood type B

42. Polygenetic Inheritance • characteristics are due to action of multiple alleles. • many genes define a trait • Height • combination of genes for height of face, size of vertebrate & length of leg bones • intelligence & happiness are result of several genes • skin color is due to interactions between at least 3 pairs of alleles • continuous traits • show gradations • there is a series of measurable intermediate forms between 2 extremes

43. Polygenetic Inheritance & Environment

44. Linked Genes • sometimes, predictions for dihybrid crosses based on Mendel's principles are violated • number of offspring obtained for each phenotype is significantly different from 9:3:3:1 ratio

45. Linked Genes • when this occurs-usually because alleles for a given trait are found on same chromosomes • during crossing over during prophase I genes always cross together • genes are said to be linked • genes located close together on chromosomes tend to be inherited together • freckles & red hair

46. Sex-Linked Genes • characteristics found on X & Y chromosome • inherited differently • X linked, recessive shows effect more in males • Recessive • no corresponding gene on Y chromosome • therefore trait will be expressed

47. Chromosomes • every nucleus in every somatic or body cell carries genetic blueprint for who we are • 46 chromosomes • each paired with a like chromosome • 23 pairs • 23 chromosomes came from our mothers • 23 from our fathers

48. Sex Chromosomes • exception found with sex chromosomes • XY chromosomes • other 22 pairs are autosomes • sex chromosomes determine gender • XX = girl & XY = boy

49. Sex-Linked Traits • sex linkage • results from action of genes present on sex chromosomes • Most are located on X chromosome • nearly all are recessive • most X-linked genes have no homologous loci on Y chromosome • baldness, color blindness & hemophilia • occur more in males than females • males receive only one allele of a gene located on t X chromosome • therefore even recessive alleles will be expressedin males • there is no dominant gene to mask it

50. Inheritance of Sex-Linked Genes • for sex linked types of traits-females are carriers if have one recessive allele • affected when possess 2 recessive alleles • sex linked characteristics follows predictable patterns of inheritance dependent on sex of offspring • affected fathers pass X-linked allele to all daughters but not to sons • males receive X chromosomes only from mothers • mothers can pass sex-linked alleles to both sons & daughters • unaffected males do not carry defective gene • carrier female has 50% chance of producing affected son • 50% chance of producing carrier daughter • affected females are homozygous-rare • condition requires both carrier mom and father with the condition